This invention relates to the field of heat exchange. The invention provides a low-cost structure, capable of tolerating high operating-temperatures, comprising a heat-exchanger or reactor such as is typically used in fuel processing or heat recovery for fuel cell systems.
In a fuel cell system, heat exchangers are typically provided to recover waste heat from a hot exhaust stream, typically 500-1000° C., and to transfer the recovered heat to one of the inputs to the system, such as fuel, air, or steam. In addition, heat exchangers that contain catalytic coatings are used as fuel processing reactors. Each system may have a unique configuration, but virtually all such systems can be made more efficient by the appropriate use of heat exchangers. In general, there is a need for a low-cost heat exchanger that can tolerate the above-described high-temperature environment, and which can be provided in large quantities, so that heat exchangers can be installed at multiple locations within a facility, at a reasonable cost. Such a heat exchanger has even more utility if one or more catalytic coatings can easily be applied to its working surfaces.
One way to limit the cost of a heat exchanger is to use a less expensive material in the manufacturing process. The use of metal foil materials, having a thickness in the range of about 0.001-0.010 inches, reduces expense by using less material overall. However, foil materials are difficult to seal or weld using conventional processes. Furnace brazing may be used to join certain high-temperature foil materials that contain nickel. Alloys that may be easily brazed include the 300 series stainless steel family (i.e. alloys known by the designations 304, 316, 321, etc.), the Inconel family (having designations 600, 601, 625, etc.), and other exotic alloys (Hastelloy-X and Haynes 230, for example). (Inconel is a trademark of Huntington Alloys Corp., of Huntington, W. Va.) These brazable alloys are always expensive because they contain nickel. To limit the cost of material, it is highly desirable to use a high-temperature foil alloy that does not contain nickel.
A desirable choice is the product known as Fecralloy, which contains iron, chromium, and aluminum (Fecralloy is a now-cancelled trademark, formerly registered by the United Kingdom Atomic Energy Authority). Fecralloy is quite inexpensive, relative to other high-temperature alloys, but it is difficult to braze. Because Fecralloy contains aluminum, the application of heat causes aluminum oxide to form, making it difficult to seal the structure by brazing.
The above problem encountered with Fecralloy can be at least partly overcome by choosing a thicker material, and using a conventional welding process. But increasing the thickness of the material increases the cost of the product, and therefore offsets the cost advantage obtained by the choice of Fecralloy.
The heat exchanger of the present invention provides a solution to the above-described problems, by providing a high-temperature heat exchanger that is both effective and inexpensive. The present invention makes it economically feasible to place heat exchangers at multiple points in a fuel cell system. The present invention could also be used in other industrial applications, such as in chemical plants.
The heat exchanger of the present invention may also be used in a steam reforming process, in which hydrocarbons are converted to hydrogen, for use in operating a fuel cell. In this process, the heat of catalytic combustion on one side drives the catalytic reaction of steam and fuel on the other side. A steam reforming process is described in US 2004/0060238 A1, US 2006/0008414 A1, and U.S. Pat. No. 7,179,313, the disclosures of which are incorporated by reference herein. The above-cited applications show various uses of heat exchange, such as in conducting an endothermic steam reforming reaction on one side of a metal strip and an exothermic combustion reaction on the other side, or in conducting a water-gas shift reaction. In general, the operation of a fuel cell presents many situations in which heat from an exothermic reaction, or from a hot exhaust source, can be used to heat some other fluid stream. In the reforming process, a single catalyst can be used for both reactions. By switching the routing of the fluids, each side of the heat exchanger can alternate between the reforming and combustion reactions. During reforming the catalyst is gradually deactivating by coking and other mechanisms, but it is regularly regenerated by the combustion duty. The heat exchanger can also be used to support other endothermic or exothermic reactions, such as water-gas shift, selective oxidation of carbon monoxide. It may also be used to support adsorbing processes such as the removal of sulfur from diesel or jet fuel.
The heat exchanger of the present invention is also compact, making it convenient for use in systems where a large amount of space is not available. The heat exchanger of the present invention also has the advantage of being hermetically sealed, so that there is virtually no possibility of leakage.